package april.test_4_9;

import java.util.Scanner;

public class TestMain1 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int m = scanner.nextInt();
        int[][] arr = new int[n + 1][m + 1];
        int k = scanner.nextInt();
        for (int i = 0; i < k; i++) {
            int x = scanner.nextInt();
            int y = scanner.nextInt();
            arr[x-1][y-1] = -1;
        }
        int[][] dpz = new int[n + 1][m + 1];
        int[][] dpf = new int[n + 1][m + 1];
        dpz[0][1] = 1;
        dpf[0][1] = 1;
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= m; j++) {
                if(arr[i - 1][j - 1] != -1) {
                    dpf[i][j] = dpf[i - 1][j] + dpf[i][j - 1];
                }
                dpz[i][j] = dpz[i - 1][j] + dpz[i][j - 1];
            }
        }
        System.out.printf("%.2f",(double) dpf[n][m] / (double) dpz[n][m] );
    }
}
